The invention relates to a portable telephone device which transmits and receives voices through a wireless line.
In a conventional portable telephone device, when a receiving error occurs due to deterioration of an input signal because of deterioration of a wireless line or the like in its quality, a channel decoding unit detects and corrects the error. In the case of a small number of receiving errors, a received data is recovered by error correction, and an encoded voice data is transmitted to a voice decoding unit as a correct voice frame. In the case of no receiving error, the encoded voice data is naturally transmitted from the channel decoding unit to the voice decoding unit as a correct voice frame.
In the case where many receiving errors are included, the channel decoding unit cannot properly perform the error correction. The channel decoding unit performs only the error detection, and transmits the error detection information to the voice decoding unit. The voice decoding unit does not use voice frame information corresponding to the error detection information, so that any abnormal tone cannot be output. An example of such a prior art portable telephone device is shown in FIG. 2. FIG. 2 is a simplified block diagram showing the configuration of a portable telephone device of the prior art.
First, referring to FIG. 2, the configuration of the conventional portable telephone device will be described. In FIG. 2, reference numeral 21 designates the portable telephone device. The portable telephone device 21 comprises an RF signal inputting unit 2 such as an antenna for receiving an RF signal 3 by radio; an RF signal processing unit 4 for converting the received RF signal 3 into a baseband signal 5; a channel decoding unit 6 for performing a channel decoding operation on the baseband signal 5 and outputting an encoded voice data 7 for each frame, for detecting and correcting an error of a voice frame, and for outputting frame error information 8 for an error which cannot be corrected; a voice decoding unit 22 for decoding the encoded voice data 7, and outputting an analog voice signal 18; and a voice generating device 19 such as a loudspeaker which generates a voice based on the analog voice signal 18.
Next, with reference to FIG. 2, the operation of the conventional portable telephone device will be described. In the portable telephone device 1, the RF signal 3 of various conditions such as fading, multipulse, or a weak field is input through the RF signal inputting means 2 to the RF signal processing means 4. The RF signal 3 is converted into the baseband signal 5 in the RF signal processing means 4, and then into the encoded voice data 7 by a channel decoding operation in the channel decoding means 6.
When the baseband signal 5 including a bit error is input, the channel decoding means 6 detects and corrects the error. If the channel decoding means detects an error which cannot be corrected, the channel decoding means supplies the frame error information 8 to the voice decoding means 22. When the voice decoding means 22 receives the frame error information 8, the voice decoding means does not use the encoded voice data 7 which corresponds to the frame error information 8, as the voice frame information. Even if the encoded voice data 7 in which an error is not corrected is received, therefore, any abnormal tone will not be generated.
In the conventional portable telephone device, however, redundant bits used in the error detection in the channel decoding operation performed by the channel decoding means 6 have a finite length, so that a frame error cannot be detected in some cases. In such cases, the frame error information 8 is not generated, and hence the encoded voice data 7 in the corresponding erroneous frame is used in the voice decoding processing in the voice decoding means 22. Based on an erroneous analog voice signal 18 which is output as a result of the above, an abnormal tone is disadvantageously generated.